In conventional wireless communication systems, voice transmissions between a base station (BS) and a mobile station (MS) (e.g., a cellular and/or dispatch handset) are typically encoded (e.g., compressed) to make efficient use of the assigned spectrum for wireless communications. On the other hand, voice transmissions across the wired network are typically not encoded (e.g., uncompressed). Such conventional wireless communication systems typically include a transcoder and rate adapter unit (TRAU) to encode voice data for transmission via the wireless medium, and decode voice data for transmission via the wired network. The following example further illustrates this aspect of conventional wireless communication systems.
FIG. 1 illustrates a block diagram of a conventional wireless communication system 100. The system 100 includes a first mobile switching center (MSC) 102, a first TRAU 104, and a first base station 106 coupled together by way of a wired communications network. The system 100 also includes a second MSC 110, a second TRAU 112, and a second base station (BS) 114 coupled together by another wired communications network. The first and second MSCs 102 and 110 may be coupled to a public switched telephone network (PSTN) 120. In this example, base station (BS) 106 is assigned to communicate with MS 108 via the wireless medium, and base station 114 is assigned to communicate with MS 116 via the wireless medium.
When voice data is sent from the originator MS 108 to the target MS 116, the voice data is first encoded (e.g., compressed) by the originator MS 108 and then sent to the base station 106 via the wireless medium. The base station 106 then sends the encoded voice data to the first TRAU 104. The transmission of the encoded voice data from the MS 108 to the base station 106 and subsequently to the first TRAU 104 is typically at a rate lower than the full voice transmission rate (e.g., typically less than the 64 kbps full voice rate). The first TRAU 104 then decodes (e.g., decompresses) the voice data, and sends it to the first MSC 102 at the full voice transmission rate (e.g., 64 kbps).
The first MSC 102 forwards the decoded voice data to the second MSC 110 at the full voice transmission rate (e.g., 64 kbps). This transmission may be made by way of a private network, or the PSTN 120. The second MSC 110 then forwards the decoded voice data to the second TRAU 112 at the full voice transmission rate (e.g., 64 kbps). The second TRAU 112 encodes (e.g., compresses) the encoded voice data and sends it to the target MS 116 by way of the base station 114 at a rate lower than the full voice transmission rate (e.g., <64 kbps).
It was noted in the industry that the conventional wireless communication system 100 has several drawbacks. First, if the originator MS 108 and base station 106 use the same encoding for voice transmission as that of the target MS 116 and base station 114, the decoding and encoding processes performed respectively by the first and second TRAUs 104 and 112 are unnecessary. Second, in such case, the decoding and then re-encoding of the voice data unnecessarily uses processing power of the first and second TRAUs 104 and 112. Third, the decoding and then re-encoding of the voice data degrades the voice quality of the delivered speech.
As a result of these drawbacks, tandem free operation (TFO) was developed to eliminate the decoding and encoding performed by the first and second TRAUs 104 and 112 under certain condition. That is, if it is determined that the encoding protocol of the voice transmission of the originator MS 108 and base station 106 is the same as that of the target MS 116 and base station 114, then the first and second TRAUs 104 and 112 may be configured to automatically eliminate the decoding and encoding processes, respectively, when it is not necessary to do so. This process is typically dynamic, such as in cases when during a voice call, the TRAUs 104 and 112 have to change from encoding-enabled mode to encoding-disabled mode or vice versa. For example, a voice call can begin in tandem free operation (TFO) mode, but when the # key of the originator MS 108 is pressed, the originator MS 108 sends out a dual tone multi-frequency (DTMF) signal intended for the target MS 116. In such case, the TRAUs 104 and 112 disable the tandem free operation (TFO) so that the DTMF signal is sent across the network and to the target MS 116 in an uncompressed format.
In order to operate under tandem free operation (TFO) mode, the TRAU 104 still uses full voice transmission rate (e.g., 64 kps) medium to send compressed voice data across the network. For each octet of data, only a portion of each octet contains voice data information. The remaining portion of each octet is populated with filler bits containing no information. For example, if the encoded voice data bit rate is 16 kbps, then only two (2) bits out of the eight (8) bits in each octet contain voice information. The originator and target TRAUs 104 and 112 exchange TFO protocol information by embedding protocol data in the two useful bits of each octet transported between the two TRAUs.
Protocols for TFO have been developed for GSM- and CDMA-based networks (See GSM 02.53, Tandem Free Operation (TFO) Service Description, version 8.0.1 Release 1999; and 3GPP2 A.S0004-B v2.0, CDMA tandem Free Operation, Aug. 5, 2002).
Wireless communication systems are now being developed to provide voice transmissions via an Internet Protocol (IP) packet network. Accordingly, instead of the voice transmission, in this example, being sent from the first MSC 102 to the second MSC 110 by way of a private network or the PSTN 120, such transmission would be by way of an IP packet network. It would be desirable for such IP network-based wireless communication systems to have TFO capability. It would also be desirable for the voice transmission across the IP network to be made in an efficient manner and independent of the TFO protocol used by the TRAUs.